1. Field of the Invention
This invention relates to bearing devices for electric motors, and more particularly to a bearing device which is suitably applicable to a capstan motor to support its shaft to which a side pressure is applied.
2. Related Art
A conventional bearing device of this type is designed as shown in FIG. 5. In FIG. 5, reference numeral 1 designates a bearing holder in the form of a hollow cylinder; and 2, the hollow of the bearing holder 1. A ball bearing 4 and an oil-impregnated bearing 3 are fixedly fitted in both end portions of the cylindrical bearing holder 1, so as to rotatably support a rotary shaft 5.
Let us consider the case where the bearing device is applied to a capstan motor adapted to drive a magnetic tape in an audio tape recorder or a VTR (video tape recorder); that is, the rotary shaft 5 is employed as the capstan shaft of the recorder. When, in this case, the pinch roller 6 of the recorder abuts against the upper end portion of the rotary shaft 5 protruded through the oil-impregnated bearing 3, to apply a side pressure to it, then as shown in FIG. 6 the rotary shaft is inclined, thus being brought into point or linear contact with both end faces of the oil-impregnated bearing 3. Under this condition, it is difficult to smoothly rotate the rotary shaft 5. Hence, as the case may be, aging is given to the rotary shaft and the oil-impregnated bearing; that is, the rotary shaft 5 is forcibly rotated for a predetermined period of time so as to grind the oil-impregnated gear 3 until the former 5 is in area contact with the latter 3.
However, the aging of the rotary shaft 5 and the oil-impregnated bearing 3 until they are brought into area contact with each other takes a considerably long period of time, thus lowering the productivity. In order to eliminate this difficulty, a bearing device as shown in FIG. 7 has been proposed in the art. That is, in the bearing device, a gear supporting annular recess 12 is formed in one end face of the bearing holder 10 in such a manner that it merges with the hollow 2. An oil-impregnated bearing 14 (FIG. 8) with a shaft-inserting hole 13, into which the rotary shaft is to be inserted, is fixedly fitted in the bearing-supporting cylindrical recess 12 in such a manner that its shaft-inserting hole 13 is in alignment with the central axis P of the bearing holder 10, and the cylindrical surface of the shaft-inserting hole 13 is slightly inclined with respect to the central axis P of the bearing holder 10.
In the above-described bearing device, the cylindrical surface of the shaft-inserting hole 13 of the oil-impregnated bearing 14 is slightly inclined with respect to the central axis P of the bearing holder 10. Hence, when the rotary shaft is inclined by the side pressure, the cylindrical surface of the shaft-inserting hole 13 of the oil-impregnated bearing 14 and the rotary shaft are quickly brought into area contact with each other; that is, the aging is achieved quickly.
The above-described conventional bearing device is advantageous in that it can be used from the beginning with the rotary shaft and the oil-impregnated bearing in good area contact with each other. However, since the cylindrical surface of the bearing-supporting cylindrical recess 12 of the bearing holder 10 in which the oil-impregnated bearing 14 is set, is slightly inclined with respect to the central axis P of the bearing holder 10, the bearing device is still disadvantageous in the following point: Because of the above-described structure, in forming the bearing holder by resin molding, or by aluminum die casting or zinc die casting, the movable pin of a metal mold (not shown) for forming the bearing-supporting cylindrical recess 12 must be moved while being kept inclined. However, with an ordinary metal mold, it is difficult to move the movable pin in this manner. Thus, the formation of the bearing holder requires considerably high technique, and accordingly an expensive apparatus. In addition, for the same reason, the productivity is low.